The understanding of many unanswered questions in immunohematology will be greately enhanced by improved techniques for quantitating erythrocyte coating with immunoglobulin and complement components. We have developed methods for 125I radioactive antiglobulin testing with anti-IgG, anti-C4 and anti-C3. These methods have proven highly satisfactory technically with only one significant drawback. The results obtained indicate uptake of labeled antiserum and not the actual amount of immunoglobulin or complement component coating the test cell. We are currently working on modifications of the 125I anti-IgG technique to permit translation of 125I anti-IgG uptake into actual amounts of IgG coating of red cells. If successful, similar modifications will be applied to 125I anti-C4 and anti-C3 testing methods. We plan to apply these quantitative techniques to studies of paroxysmal nocturnal hemoglobinuria (PNH), auto-immune hemolytic anemia (AIHA), and general studies in immunohematology. In PNH we plan to study the uptake of C4 and C3 by normal, PNH, and artificial PNH cells to delineate mechanisms of hemolysis in various systems. In AIHA we plan to quantitate erythrocyte coating in patients with positive direct antiglobulin tests (with and without hemolysis) and patients with apparent immune hemolytic anemia and negative antiglobulin tests. A number of general studies are planned, including the use of the radioactive antiglobulin technique to determine Rh phenotype in patients with positive direct antiglobulin reactions.